Changing landscape A new study says Melbourne's Port Phillip Bay was dry ground with a couple of meandering rivers a thousand years ago.

Scientists warn the discovery means the area's recent decade-long drought, which saw a 25 per cent reduction in rainfall, combined with any future silting up of the bay's entrance 'The Rip' could tip the bay into another dry period within 50 years.

The ancient river beds were detected during surveys by the Port of Melbourne as part of its shipping channel dredging program.

Holgate says each channel was about five metres deep and a hundred metres wide.

"There had to be something going on with the water level in the bay, independent of sea levels which have been stable for the past six or seven thousand years", says Holdgate.

"The river channels were flowing from the Yarra and Werribee Rivers into a small lake about 20 metres below sea level in the southern part of the bay."

"Carbon-14 dating of shells in core samples put the drying to between 2800 and 1000 years ago."

Holdgate says it may have been caused by a sand bar, probably created by storms, blocking the sea entrance, combined with drought and high evaporation rates."

The scientific evidence also fits with anecdotal evidence from Aboriginal oral history indicating the area was once dry land.

"Aboriginal oral history talks about it happening very quickly during a great flood," says Holdgate.

"The traditional story involves a group of young boys being left behind at camp while the adults were looking for food. The boys threw a spear into a magic water container which burst, causing the area to suddenly flood."

Not the first time

Holdgate says marine shell beds up stream at Flemington indicates another event about 5500 years ago, when water levels in the bay were up to two metres higher than now.

"That could have meant higher sea levels at the time or another blockage of the entrance," says Holdgate.

"There's also evidence of a drying event during the last glaciation about 10,000 years ago when sea levels were lower than they are today".

But Holdgate says the ongoing dredging program means a dry Port Phillip Bay isn't it likely to happen soon. But if it did, the result could be unsavoury for locals.

"We can see the impact of such events in places like Lake Alexandrina on the Murray mouth," he says. "It would mean the bay becoming more saline, smelly and unsustainable for many birds and marine animals."